1. Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM) passive optical network (PON) system, and more particularly, to an external cavity laser (ECL) used as a light source in the WDM PON system.
2. Description of the Related Art
In a general wavelength division multiplexing (WDM) system, the denser the channel spacing, the greater the number of channels. Accordingly, the allowable range of wavelength error of a light source that may occur with respect to the external temperature is strictly limited in the WDM system. In most WDM systems, distributed feedback-laser diode (DFB-LD) optical transmission modules are used, which are prepared in an expensive butterfly type package in which temperature controlling components such as a thermoelectric cooler (TEC) or a thermistor are mounted.
For example, in an optical transmission module used for dense wavelength division multiplexing (DWDM) having a channel spacing of 50 GHz to 100 GHz (0.4 nm or 0.8 nm), the oscillation wavelength should be stabilized such that crosstalk due to interference with adjacent channels, without being affected by the variation of external temperature, is not over an allowable range, and moreover, the optical output passing through the wavelength band of multiplexing/demultiplexing used in the network should be stabilized. For this purpose, a butterfly package in which temperature controlling components such as a TEC, a thermistor, etc. are mounted is used to keep the temperature of an optical device uniform.
However, since controlling components such as a TEC, a thermistor, a heat dissipating plate, a temperature controlling circuit, etc. need to be additionally attached to the DFB lasers, and an expensive butterfly type package should be selected, the amount by which the price of the optical transmission module can be reduced reaches a certain limit. Also, since it is difficult to apply the optical transmission modules including a monitoring photodetector and a wavelength fixing unit to a PON, the greatest advantage of which is its price competitiveness, a monitoring photodetector and a wavelength fixing unit should be added to stabilize the oscillation and the optical output, and this raises the price of the optical transmission modules including a monitoring photodetector and a wavelength fixing unit. Therefore, in order to replace the expensive DFB-LD, an external cavity laser is suggested as a light source.
FIG. 1A is a cross-sectional view of a conventional external cavity laser (ECL) using a Bragg grating in a TO-CAN package.
Referring to FIG. 1A, the ECL includes a semiconductor amplifier 10 as an amplifying material, a focusing lens 20, and an optical fiber 30 of a core 32 in which a Bragg grating 34 is formed and a cladding 36 surrounding the core 32. The Bragg grating 34 of the optical fiber 30 is stable against temperature variations, and a rear surface 11 of the semiconductor amplifier 10 and the Bragg grating 34 form an ECL. Referring to FIG. 1A, the length of the external cavity Lcavity forming an external cavity is denoted as a double-sided arrow.
The optical fiber 30 including the Bragg grating 34 is fixed in a ferrule 50 by thermosetting epoxy 40 to form an optical fiber structure 60 forming a TO-CAN package. The rear surface 11 of the semiconductor amplifier 10 is generally high reflection (HR)-coated, and a front surface 12, that is, an exit surface, may be anti-reflection (AR)-coated.
The focusing lens 20 is used to improve the optical combination efficiency of the semiconductor amplifier 10 and the optical fiber 30. A cross-section 37 of the optical fiber 30 is inclined at a predetermined angle to the perpendicular of an optical path Poptic to reduce residual reflection on the cross-section 37. The optical path Poptic of the laser is denoted with a dotted line in a cavity 70. When an external cavity is formed in the above structure, wavelengths reflected in the Bragg grating 34 among the wavelengths satisfying the phase matching conditions are oscillated and then output.
FIG. 1B is a cross-sectional view illustrating a portion I-I of FIG. 1A, wherein the core 32, the cladding 36, the thermosetting epoxy 40, and the ferrule 50 are concentrically stacked.
The wavelength stability with respect to the external temperatures of the ECL in which the DFB-LD is TO-CAN packaged is as follows. In the case of the DFB laser in which the grating determining the oscillation wavelength is in a semiconductor gain region, the manufacturing cost is low, but when the temperature is not controlled, the thermooptical coefficient (∂nLD/∂T) of the semiconductor material is about 2.4×10−4/K, and the oscillation wavelength varies by 0.1 nm per 1° C. according to Equation 1, where nLD refers to the refractive index of the semiconductor material of the DFB laser.∂λ/∂T=λ(∂n/∂T)/n  [Equation 1]
In order to improve the wavelength stability of the light source, in the case of an ECL in which the grating is carved not in a semiconductor gain material but in an optical fiber having a low thermooptical coefficient, the wavelength stability can be improved to 0.01 nm per 1° C.
However, examining the wavelength spectrums with respect to the temperature when the ECL oscillates in a single mode, mode hopping occurs. Here, mode hopping refers jumping of oscillation wavelengths at a predetermined external temperature.
FIGS. 2A through 2C are graphs illustrating mode hopping when the ECL oscillates in a single mode.
FIG. 2A is a graph showing external cavity modes 92 determined by the phase matching condition of the external cavity and the reflection spectrums 94 determined by the Bragg grating of the optical fiber. The graphs show that an m-th mode sensing the highest reflectivity in the reflection spectrums 94 among the external cavity modes 92 is the oscillation mode 90 of the ECL.
FIG. 2B is a graph showing the external cavity modes 110 and the reflection spectrums 120 varying according to the increase of temperature. Referring to FIG. 2B, when a current is applied for a long time or the temperature of the ECL is increased due to changes in the external environment, the external cavity modes 92 and the reflection spectrums 94 change to first shifting external cavity modes 93 and first shifting reflection spectrums 95, and accordingly, the oscillation modes 90 of the ECL change to first shifting oscillation modes 91.
Here, a shifting distance (Δm1) of the external cavity modes is represented by ∂λECL/∂T, and the shifting distance is determined by the thermooptical coefficient of the materials forming the ECL and the length of the optical path according to Equation 2.∂λECL/∂T=λ(Σ(∂ni/∂T)Li)/Σ∂niLi  [Equation 2]
Also, since a shifting distance (ΔR1) of the reflection spectrum is proportional to the thermooptical coefficient of the light waveguide in which the Bragg grating is carved, thus the shifting distance (ΔR1) of the reflection spectrum can be represented by ∂λWBG/∂T. The index WBG refers to a waveguide Bragg grating. Accordingly, in a predetermined temperature range, the first shifting oscillation mode 91 of the final output wavelength becomes the m-th mode, since the shifting distance (ΔR1) of the reflection spectrum is similar to the shifting distance (Δm1) of the external cavity modes, and the mode sensing the highest reflectivity is the m-th mode.
In FIG. 2C, the ECL has a higher temperature than in FIG. 2B, and the external cavity modes 92 and the reflection spectrums 94 are respectively changed into second shifting external cavity modes 93a and second shifting reflection spectrums 95a. However, since an (m−1)th mode of the oscillation modes senses greater reflectivity than an m-th mode, the oscillation mode of the final output wavelength of the ECL does not become the first shifting oscillation mode 91a but a second shifting oscillation mode 96. Thus, a change in an oscillation mode according to external temperature changes is called mode hopping (Hm), and such mode hopping happens periodically according to external temperature changes.
FIG. 3 is a graph showing the movement of the oscillation wavelength according to the temperature and mode hopping which occurs periodically according to the temperature changes.
Referring to FIG. 3, the oscillation wavelength moves to the long wavelength band gradually as the temperature of the ECL increases, and the mode hopping occurs at points with a regular period 97 determined by Equation 3.ΔT=δλECL/[(dλ/dT)ECL−(dλ/dT)WBG]  [Equation 3]
The wavelength interval of the mode hopping is an interval between the external cavity modes determined by the optical path of the external cavity. The wavelength change 99 according to the temperature of the Bragg grating is ∂λWBG/∂T, and the wavelength change 98 according to the temperature of a substantial oscillation wavelength due to mode hopping is ∂λECL/∂T.
In the case of the ECL operating in a single mode, the output optical power of the ECL in the mode hopping region in which the oscillation wavelength is rapidly converted is known to change by 50% or more. Such rapid change in optical power not only rapidly deteriorates the transmission quality of the WDM PON but also rapidly deteriorates long-term reliability of a device.
In order to solve the mode hopping of the ECL operating in a single mode, the reflection spectrum of the Bragg grating should be kept regular regardless of temperature, and the external cavity modes should not be affected by changes in temperature.
Meanwhile, in the case of the ECL operating in a multi-mode, the variation in the output optical power by mode hopping is minimal compared to a single mode. This is because the total of the output optical power of each mode is kept regular even if individual oscillation modes experience rapid light output changes during mode hopping. Accordingly, the oscillation wavelengths output from the ECL operating in a multi-mode can be designed irrespectively of external temperature changes. Also, in a multi-mode, since the width of a spectrum is broader than in a single mode, it is influenced by dispersion during transmission, and thus to reduce the influence by dispersion, the spacing and number of the oscillation modes need to be controlled.